The present invention relates to a self-locking fastener, and more particularly to an earth bolt.
A conventional self-locking fastener is constructed such that crests of a smaller number of threads of the self-locking fastener than that of threads of a nut, into which the self-locking fastener is screwed, are deformed so that the threads have a cross section of a trapezoidal-shape over an arcuate region of 90 degrees about an axis of the self-locking fastener in a cross section perpendicular to the axis, and so that one of the group of the deformed threads, which is disposed at a lowermost axial end, is positioned within the nut when the fastener is screwed into the nut. The deformed thread portions are separated by approximately 180 degrees about the axis of the self-locking fastener, that is, the deformed thread portions in one thread are formed in two positions so as to face each other with the axis of the self-locking fastener therebetween. In addition, since the deformed thread portions have a somewhat smaller flank angle than that of non-deformed thread portions or normal threads, flanks of the deformed thread portions are forcedly brought into close contact with flanks of a nut having normal threads when the self-locking fastener is screwed into the nut, with the result that the deformed thread portions exhibit a looking effect. As used hereinbelow, the deformed thread portions brought into close contact with flanks of the nut are referred to as "lock portions".
The deformed thread portions in such conventional self-locking fastener occupy a half of a circumferential length of a thread which includes the deformed thread portions, so that there is a large contact area between the self-locking fastener and the flanks of a nut, into which the self-locking fastener is screwed. Accordingly, screwing the self-locking fastener into the nut requires a great force (hereinafter, referred to as "screwing torque"), resulting in problems of not only worsening the quality in a screwing operation but also generating heat due to frictional resistance in lock portions to cause seizure of the self-locking fastener onto the nut when the screwing operation is carried out at high speeds by means of an air motor or the like. Further, forced screwing may give rise to fracture of threads of the nut with the result that the self-locking fastener may fall off. As used hereinafter, a force leading to fracture of threads is referred to as "nut fracture torque". In order to avoid such problems, coating the self-locking fastener with a wax prior to the screwing work has been essential. The use of wax leads to not only an increased cost but also to a complicated screwing process.
In the case where a conventional self-locking fastener is used as an earth bolt for automobiles, it is screwed into a nut (welded to a predetermined portion of a vehicle body) which is subjected to a rust preventive treatment together with the vehicle body and surfaces of which are coated with paint. Accordingly, the paint adhered to threads of the nut must be scraped off in the screwing operation in order to ensure electrical conductivity, so that a plurality of grooves in parallel to an axis of the self-locking fastener are formed on a peripheral edge at a lower end of the self-locking fastener to perform scraping-off of the paint in the screwing operation.